1. Field of the Invention
The present invention relates to a circuit for driving a liquid crystal display device (LCD), and more particularly, to a driving circuit for dot inversion method using a line inversion mechanism and single bank mode.
2. Discussion of the Related Art
Cathode ray tubes (CRT) are widely used in display devices, such as television sets and display monitors for computers because CRTs can easily reproduce color and have high response speed. However, CRTs are too large and heavy, and consume too much power to be portable. Because of this, it is desirable to replace CRTs with other types of display. To overcome the above mentioned disadvantages of the CRT, a considerable amount of research and development has been conducted to design alternative types of display, such as liquid crystal displays, plasma display panels, and so on. Among them, a liquid crystal display is one of the most widely used devices. This is because the LCD does not have the bulky electron gun as is sed with the CRT, and the LCD can be applied to a thin television set that is mounted on the wall. Furthermore, the LCD can be applied to a portable display device, such as a note-book computer, because the power consumption is very low. Accordingly, the LCD can be driven by a battery.
The schematic structure of a conventional LCD is shown in FIGS. 1 and 2. FIG. 1 shows the perspective view, and FIG. 2 shows the structure of the lower panel. The LCD includes an upper panel 21, which has a polarization plate 20, a color filter 22, and a common electrode 23; a lower panel 25, which has thin film transistors (TFTs) 13 and pixel electrodes 26; and a liquid crystal material 24 inserted between the upper panel 21 and the lower panel 25. The lower panel 25 further includes a plurality of scan lines 14 and a plurality of data lines 15. The scan lines 14 and the data lines 15 perpendicularly cross each other. At the area surrounded by the neighboring scan lines and data lines, the pixel electrode 26 is formed. At each of the intersections of the scan lines and data lines, the TFT 13 is formed. Each of the area surrounded by the neighboring scan lines and data lines is called a pixel. Thus, one pixel includes the pixel electrode 26, the common electrode 23, and the liquid crystal material 24 in between. In addition, the lower panel 25 has a data driver IC 11 connected to the data lines 15 and a scan driver IC 10 connected to the scan lines 14 (FIG. 2).
The TFT includes a gate electrode, a source electrode and a drain electrode. The gate electrode is connected to the scan line, the source electrode is connected to the data line, and the drain electrode is connected to the pixel electrode. The drain electrode and the source electrode are connected with a semiconductor layer. The TFT works as a switch that passes a data voltage applied to the data line to the drain electrode when a scan voltage is applied to the gate electrode through the scan line. The data voltage applied to the drain electrode is applied to the pixel electrode connected to the drain electrode.
As shown in FIG. 2, video data is applied from a controller 17 to the data driver IC 11. The video data includes grey scaled data of red (R), green (G), and blue (B), which are applied to the corresponded pixel electrodes 26. The data driver IC 11 latches the video data that come from the controller IC 17 until all the data of one line are inputted. Then, the video data of one line is transferred to the data line at once. At that time, the scan driver IC 10 applies a scan voltage to the scan line 14 connecting TFTs 13 to reproduce the video images at the pixel electrodes according to the scan signal of the controller 17.
An example of the TFT is explained with reference to FIG. 3. FIG. 3 shows a cross-sectional view of the TFT. A gate electrode 30 is formed on the lower substrate 25, and a gate insulating layer 31 is formed thereon. An active layer 34 made of amorphous silicon or polysilicon, for example, is formed on the gate insulating layer opposite the gate electrode 30. Source and drain electrodes 32, 33 are connected to both sides of the active layer 34 through an ohmic contact layer 36 (or n.sup.+ layer). A protective layer 35 is formed over the resultant structure. Finally, pixel electrode 26 made of transparent conductive material, such as indium tin oxide (ITO), is formed on the protective layer 35 to be connected to the drain electrode 33 through a contact hole made in the protective layer 35.
When the scan voltage is applied to a scan line, all the TFTs connected to the scan line are turned on. Accordingly, the video data applied to the data lines are sent to the pixel electrode through the TFTs. Therefore, a voltage is applied to each pixel electrode. On the other hand, constant voltage is applied to the common electrode. Accordingly, a voltage difference is formed between the pixel electrode and the common electrode, and an electric field is formed by the voltage difference. The arrangement (or orientation) of the liquid crystal molecules between the pixel and common electrodes is changed according to the electric field, modulating the amount of light transmission at the pixel. Thus, there are differences in light transmission between the pixel applied with a data voltage and the pixel not applied with a data voltage. Using these properties of pixels, the LCD works as a display device.
Generally, an LCD uses one of the line inversion, the column inversion, and the dot inversion methods, according to the phase of the applied signal voltage. In the line inversion, as shown in FIGS. 4a and 4b, the polarity of voltage applied to the pixel electrodes is reversed at every scan line (row). In the column inversion, as shown in FIGS. 5a and 5b, the polarity of voltage applied to the pixel electrodes is reversed at every data line (column). In the dot inversion method, as shown in FIGS. 6a and 6b, the polarity of voltage is reversed at every row and column. FIGS. 4a, 5a, and 6a represent the phases of the common electrode voltages in a particular frame, and FIGS. 4b, 5b, and 6b represent the phases of the pixel electrode voltages in the same frame. In the next frame, these phases are reversed. The reason for changing the phase of signal is that if the applied voltages to the common and pixel electrodes are the same value in the entire respective electrodes, then the liquid crystal is heated, and the quality of the picture screen deteriorates.
In the line and column inversion method, a flicker problem occurs. The reason is the following. When a scan line signal is "HIGH," all the TFTs connected to the scan line are turned on, and the data signals are sent to the pixel electrodes from the source electrodes connected to the data lines. Then, the liquid crystal is driven by the voltage difference between the pixel electrode and the common electrode. When the scan line signal is "LOW", all the TFTs connected to the scan line are turned off. At that time, the voltage applied to the pixel electrodes remains in the pixel electrode, so the liquid crystal is still in the same condition, and the display signals are maintained. However, the stored signal voltage in the pixel electrode is reduced by .DELTA.V by coupling capacitors (Cgs), which are formed between the scan lines and data lines. Since the voltage in the pixel electrodes are not maintained constant, the display has a flicker problem.
In the dot inversion method, the flicker problem does not occur because the neighboring pixels have the different polarity in signal. If the first pixel is applied with a positive signal, the second pixel (neighboring pixels) is applied with a negative signal. At the next period, the first pixel has a negative signal and the second pixel has a positive signal. That is, the pixel signal is a pulse signal type. The voltage charge .DELTA.V, which occurs in positive and negative states, can be moderated by controlling the common voltage. Therefore, the voltage differences can be maintained constant, solving the flicker problem.
An example of the dot inversion method is described with reference to FIGS. 7, 8, and 9. See also Japanese Patent Laid Open Publication No. 63-229495. A plurality of common electrodes are disposed in parallel with the data bus lines. The common electrodes are grouped into two groups. One group is constructed by the mutually connected odd-numbered common electrodes, and the other group is constructed by the mutually connected even-numbered electrodes. The odd-numbered common electrodes are applied with the first common voltage Com1, and the even-numbered common electrodes are applied with the second common voltage Com2. On the lower panel, the odd data lines are connected to the first driver IC DD.sub.1 disposed at the upper side of the lower panel, and the even data lines are connected to the second driver IC DD.sub.2 disposed at the lower side of the lower panel (FIG. 8). The data signals output from the first driver IC DD.sub.1 have phases 180-degree different from that of the second driver IC DD.sub.2. This structure, having the driver ICs at the two sides, is called a double bank mode. On the other hand, the structure, having the driver ICs at only one side, is called a single bank mode.
In order to adopt the dot inversion method for driving an LCD, the structure of the LCD is designed in double bank mode. Because inverters 93, such as "NOT gate," should be disposed in every other output terminal of the data driver IC 90, as shown in FIG. 9, designing the dot inversion method in a single bank mode is very complicated and difficult. However, in the double bank driving method, the visible area of the display panel is smaller than the single bank mode because the driver ICs are disposed at the two sides of the panel. In the COG (Chip On Glass) technique, this problem is more serious.